My camping/playing truck (which also serves as my "property management" work rig), is an '02 F350 diesel crew cab long bed SRW. It has the OEM camper package and towing package. The rear springs have a 2" block/spacer (OEM for the SRW F350 and the sole difference between an OEM F350 and a F250, or so I've read) so it sat higher in the rear as stock. About 6 years ago I had a so-called "leveling kit" installed on the front. The kit included new front leaf springs, U-bolts, bushings, and all related hardware. The OEM front springs were toast by then, likely from supporting 1,200 lbs of diesel engine, so I needed something other than simple blocks or spacers. I had the 4-Wheel Parts store here in Raleigh, NC do the work and of course the kit was purchased through them.
Since then, and in anticipation of heavier loads in the bed and/or towed tongue weight, I purchased small bolt-on blocks which add to the short "overload" leaf, itself situated over the main rear leaf spring pack. The bolt-on blocks have the effect of contacting the overload spring stop with less downward deflection of the bed, in essence causing the overload leaf to become a more active part of the suspension. It of course does not constitute a lift per se, but it does keep the ride more level when loaded. The downside of the bolt-on blocks is that it adversely affects the rear suspension's articulation, but I've got no business rock-crawling a 20' long 8,000 lb truck to begin with.
Foy